Meniscus projection plane setting apparatus, meniscus projection plane setting method, and meniscus projection plane setting program

ABSTRACT

A meniscus projection plane setting apparatus, a meniscus projection plane setting method, and a meniscus projection plane setting program appropriately set a projection plane when generating a projection image of a meniscus from a three-dimensional image including a knee joint. An image acquisition unit acquires the three-dimensional image of the knee joint. A plane setting unit sets a plane that approximates a joint surface of the knee joint as the projection plane for generating the projection image by projecting the meniscus at the knee joint included in the three-dimensional image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No.PCT/JP2020/039629 filed on Oct. 21, 2020, which claims priority under 35U.S.C § 119(a) to Japanese Patent Application No. 2019-223582 filed onDec. 11, 2019. Each of the above application(s) is hereby expresslyincorporated by reference, in its entirety, into the presentapplication.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to a meniscus projection plane settingapparatus, a meniscus projection plane setting method, and anon-transitory computer readable recording medium storing a meniscusprojection plane setting program that set a projection plane whengenerating a projection image of a meniscus from a three-dimensionalimage including a knee joint.

2. Description of the Related Art

In recent years, by the progression of medical equipment such as acomputed tomography (CT) apparatus and a magnetic resonance imaging (MM)apparatus, a high-quality, high-resolution three-dimensional image hasbeen used for image diagnosis. Since a three-dimensional image isconstituted by a large number of two-dimensional images and has a largeamount of information, it may take time for a physician to find adesired observation body part for diagnosis. Thus, the diagnosisefficiency may be increased by increasing the visibility of the wholeorgan or a lesion in the following manner. An organ of interest isrecognized and extracted from a three-dimensional image including theorgan of interest by using a method such as a maximum intensityprojection (MIP) method or a minimum intensity projection (Min IP)method to display an MIP image, or to display a three-dimensional imageby volume rendering (VR).

On the other hand, osteoarthritis is a disease that commonly occurs inelder people. In particular, knee osteoarthritis causes pain in the kneejoint and reduction in a movement range, and thus, the progression ofthe symptoms may disable the patient from walking. For diagnosis of suchosteoarthritis, knee joint cartilages need to be evaluatedqualitatively. Thus, various methods for quantifying knee jointcartilages using a three-dimensional image have been proposed. Forexample, JP2018-042709A proposes a method in which a projectiondirection of a cartilage region extracted from an Mill image isdetermined, the cartilage region is projected in the determinedprojection direction to generate a projection image, and a quantitativevalue of the cartilage region is derived in the projection image. Byusing the method described in JP2018-042709A, a region for quantifyingthe cartilage region can be determined appropriately, and thus, a stablediagnosis result for the cartilages can be obtained. In particular, byusing a quantitative value as the thickness of the cartilages, thethickness of the cartilages can be evaluated.

In addition, JP2013-533765A proposes a method in which the head center,radius, and neck axis of an articular bone are determined from athree-dimensional image of a joint in order to identify a sphericaljoint such as a femoral neck. In the method described in JP2013-533765A,a three-dimensional neck minimal curve is determined on athree-dimensional surface model of a neck portion of a bone; a leastsquares fitting plane to the three-dimensional neck minimal curve isdetermined; the orthogonal direction to the least squares fitting planeis computed as the direction of a precise neck axis; the center of theprojection of the three-dimensional neck minimal curve on the leastsquares fitting plane is computed as a point of the precise neck axis.

SUMMARY OF THE INVENTION

Although causes of the knee osteoarthritis have not been clarified yet,one of the causes of the knee osteoarthritis is thought to bedeformation and tearing of menisci, which are present at a knee joint.For example, it is thought that the knee osteoarthritis progressesbecause cartilages are lost due to deformation, tearing, or deviation ofthe menisci. Thus, it is important to evaluate the state of menisciquantitatively for diagnosis of knee osteoarthritis. To evaluate themenisci quantitatively, a projection plane for projecting the menisci ina three-dimensional image of a knee joint needs to be set appropriately.

However, the knee joint includes a curved surface. In particular,although a tibial joint appears to be flat, inclinations are differentin the medial condyle and the lateral condyle, and the joint surface hasa depressed shape. Thus, even if the method described in JP2018-042709Ais used, an error in quantifying the menisci becomes large unless theprojection direction is set appropriately. For example, as illustratedin FIG. 25, it is assumed that, out of a meniscus 91A and a meniscus 91Bon a joint surface of a tibia 90, the meniscus 91B on the joint surfaceon the lateral condyle side of the tibia 90 is projected in thedirection of an arrow A that inclines with respect to a central axis C0of the tibia 90. In this case, as illustrated in FIG. 26, if there is alost portion 92 due to damage or the like of the meniscus 91B asindicated by the oblique lines, the lost portion 92 is projected as ifthe meniscus 91B is present on a projection image 93. In addition, themethod described in JP2013-533765A determines the axis in a sphericaljoint and is not to appropriately set a projection plane for projectingthe knee joint.

The present disclosure has been made in view of the above circumstances,and an object thereof is to appropriately set a projection plane forgenerating a projection image of a meniscus from a three-dimensionalimage including a joint.

A meniscus projection plane setting apparatus according to the presentdisclosure includes at least one processor configured to: acquire athree-dimensional image of a knee joint; and set a plane thatapproximates a joint surface of the knee joint as a projection plane forgenerating a projection image by projecting a meniscus at the knee jointincluded in the three-dimensional image.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to exclude, fromamong pixel positions on the joint surface, a pixel position whosedistance from the projection plane is greater than or equal to apredetermined threshold and set a new projection plane.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to set theprojection plane by repeating excluding of the pixel position andsetting of the new projection plane a plurality of times.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to set the planethat approximates the joint surface by a least squares method.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the knee joint may be a tibial joint.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to extract, as ajoint surface region, a region excluding a region between condyles froman image in which a joint surface of the tibia is viewed in apredetermined direction and set, as the projection plane, a plane thatapproximates the joint surface included in the joint surface region.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to generate theprojection image by further projecting the meniscus at the knee joint inthe three-dimensional image in a direction orthogonal to the projectionplane.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to generate theprojection image by further projecting the meniscus at the knee joint inthe three-dimensional image in a direction orthogonal to the projectionplane such that a line connecting a center of gravity of a joint surfaceof a medial condyle of the tibia and a center of gravity of a jointsurface of a lateral condyle of the tibia is oriented in a predetermineddirection.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to generate theprojection image by further projecting the meniscus at the knee joint inthe three-dimensional image in a direction orthogonal to the projectionplane such that an Akagi line at the tibia when viewed in a directionperpendicular to the projection plane is oriented in a predetermineddirection.

The “Akagi line” is a line connecting a posterior cruciate ligamentattachment and the medial border of a patellar tendon attachment.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to further cause adisplay to display the projection image.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to further derive aquantitative value of the meniscus in the projection image.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to derive, as thequantitative value, at least one of an area of the meniscus, a volume ofthe meniscus, a lost area of the meniscus, a representative value of athickness of the meniscus, or thicknesses of the meniscus at positionsof the projection image.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to generate athickness map of the meniscus in a case of deriving the thicknesses ofthe meniscus at the positions of the projection image.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to further cause adisplay to display the thickness map.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to derive thequantitative value of the meniscus within or out of a region of interestat the knee joint in the three-dimensional image in the projectionimage.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the region of interest may be a region where asubchondral bone region or a cartilage is supposed to be present at theknee joint.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor is configured to set, in a case wherethere is a derived result of another quantitative value derived fromanother three-dimensional image whose imaging timing is different for asame test subject as a test subject for which the three-dimensionalimage is acquired, the region of interest at a same position as aposition of the region of interest in deriving the other quantitativevalue.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to derive, as thequantitative value, a coverage of the meniscus in the region ofinterest.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to derive, as thequantitative value, at least one of a volume of the meniscus out of theregion of interest, an area of the meniscus out of the region ofinterest, or a thickness of the meniscus out of the region of interest.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to divide themeniscus in the projection image into a plurality of regions and derivethe quantitative value in each of the regions obtained by dividing.

In the meniscus projection plane setting apparatus according to thepresent disclosure, the processor may be configured to set, in a casewhere there is another projection image generated from anotherthree-dimensional image whose imaging timing is different for a sametest subject as a test subject for which the three-dimensional image isacquired, a projection plane, by which the other projection image isgenerated, as the projection plane for generating the projection imageby projecting the three-dimensional image.

A meniscus projection plane setting method according to the presentdisclosure includes: acquiring a three-dimensional image of a kneejoint; and setting a plane that approximates a joint surface of the kneejoint as a projection plane for generating a projection image byprojecting a meniscus at the knee joint included in thethree-dimensional image.

It is also possible to provide a non-transitory computer readablerecording medium storing a program for causing a computer to execute themeniscus projection plane setting method according to the presentdisclosure.

According to the present disclosure, it is possible to appropriately seta projection plane for generating a projection image of a meniscus froma three-dimensional image including a joint.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hardware configuration diagram illustrating an overalldiagnosis support system to which a meniscus projection plane settingapparatus according to an embodiment of the present disclosure isapplied;

FIG. 2 is a schematic block diagram illustrating a configuration of themeniscus projection plane setting apparatus according to the embodiment;

FIG. 3 is a diagram illustrating a three-dimensional image of a kneejoint;

FIG. 4 is a diagram illustrating a three-dimensional region;

FIG. 5 is a diagram for describing extraction of joint surface regions;

FIG. 6 is a diagram for describing deriving of a projection plane;

FIG. 7 is a diagram for describing exclusion of a voxel;

FIG. 8 is a diagram for describing generation of a projection image;

FIG. 9 is a diagram for describing deriving of centers of gravity;

FIG. 10 is a diagram illustrating a projection image including a lineconnecting the centers of gravity;

FIG. 11 is a diagram illustrating the projection image including anAkagi line;

FIG. 12 is a diagram for describing setting of regions of interest;

FIG. 13 is a diagram for describing deriving of quantitative values;

FIG. 14 is a diagram illustrating a display screen of quantitativevalues of menisci;

FIG. 15 is a diagram illustrating a display screen for temporalcomparison of the quantitative values of the menisci;

FIG. 16 is a diagram for describing setting of regions of interest;

FIG. 17 is a diagram illustrating a projection image of the regions ofinterest and the menisci;

FIG. 18 is a diagram for describing deriving of the thickness of ameniscus;

FIG. 19 is diagram for describing deriving of the average thickness ofthe meniscus;

FIG. 20 is a diagram for describing dividing of meniscus regions;

FIG. 21 is a diagram for describing deriving of quantitative values ofthe menisci that are out of the regions of interest;

FIG. 22 is a diagram illustrating a thickness map of the menisci;

FIG. 23 is a flowchart illustrating a process performed in the presentembodiment;

FIG. 24 is a diagram for describing generation of a three-dimensionalprojection image;

FIG. 25 is a diagram for describing generation of a projection image ofa meniscus; and

FIG. 26 is a diagram for describing generation of the projection imageof the meniscus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, an embodiment of the present disclosure will be described withreference to the drawings. FIG. 1 is a hardware configuration diagramillustrating an overall diagnosis support system to which a meniscusprojection plane setting apparatus according to the embodiment of thepresent disclosure is applied. As illustrated in FIG. 1, in thediagnosis support system, a meniscus projection plane setting apparatus1 according to the present embodiment, a three-dimensional imagingapparatus 2, and an image storage server 3 are communicably connectedvia a network 4.

The three-dimensional imaging apparatus 2 is an apparatus that images adiagnosis-target body part of a test subject thereby generating athree-dimensional image representing the body part. Specifically, thethree-dimensional imaging apparatus 2 is a CT apparatus, an MRIapparatus, a positron emission tomography (PET) apparatus, or the like.A three-dimensional image generated by the three-dimensional imagingapparatus 2 is transmitted to the image storage server 3 and is storedtherein. Note that in the present embodiment, the diagnosis-target bodypart of a patient who is a test subject is a knee joint, thethree-dimensional imaging apparatus 2 is an MRI apparatus, and an MMimage of the test subject's knee joint is generated by thethree-dimensional imaging apparatus 2 as a three-dimensional image.

The image storage server 3 is a computer that stores and manages varioustypes of data and includes an external mass storage device and databasemanagement software. The image storage server 3 communicates with theother apparatuses via the network 4 by wire or wirelessly to transmitand receive image data or the like. Specifically, the image storageserver 3 acquires various types of data including image data of athree-dimensional image or the like generated by the three-dimensionalimaging apparatus 2 via the network and stores the image data in arecording medium such as the external mass storage device to manage theimage data. Note that the form of storage of image data andcommunication between the apparatuses via the network 4 conform to aprotocol such as Digital Imaging and Communication in Medicine (DICOM).

The meniscus projection plane setting apparatus 1 is a single computerin which a meniscus projection plane setting program according to thepresent disclosure is installed. The computer may be a workstation or apersonal computer that a physician who performs diagnosis directlyoperates, or may be a server computer connected to the work station orpersonal computer via the network. The meniscus projection plane settingprogram is stored in a storage device of the server computer connectedto the network or a network storage in an externally accessible state,is downloaded to a computer used by a physician on demand, and isinstalled. Alternatively, the meniscus projection plane setting programis recorded on a recording medium such as a digital versatile disc (DVD)or a compact disc read only memory (CD-ROM), is distributed, and isinstalled in a computer from the recording medium.

FIG. 2 is a diagram illustrating a schematic configuration of themeniscus projection plane setting apparatus according to the embodimentof the present disclosure, implemented by the meniscus projection planesetting program being installed in a computer. As illustrated in FIG. 2,the meniscus projection plane setting apparatus 1 includes a centralprocessing unit (CPU) 11, a memory 12, a storage 13, and a communicationinterface (I/F) 14 as a configuration of a standard workstation. Inaddition, a display 6 and an input device 7 such as a mouse or akeyboard is connected to the meniscus projection plane setting apparatus1.

Various types of information are stored in the storage 13. The varioustypes of information include a three-dimensional image of a test subjectand information necessary for processing, which are acquired from theimage storage server 3 via the network 4. Note that this embodimentassumes that a three-dimensional image V0 in which a knee joint of atest subject is a diagnosis-target body part is stored in the storage13.

The communication I/F 14 is a network interface for controllingtransmission of various types of information to/from an externalapparatus such as the image storage server 3 via the network 4.

In the memory 12, the meniscus projection plane setting program read outby the CPU 11 is temporarily stored. As processing to be executed by theCPU 11, the meniscus projection plane setting program prescribes animage acquisition process in which the three-dimensional image V0 of theknee joint is acquired; a plane setting process in which a plane thatapproximates a joint surface of the knee joint is set as a projectionplane for generating a projection image, which will be described later,by projecting menisci at the knee joint included in thethree-dimensional image; a projection process in which the menisci areprojected onto the projection plane and the projection image isgenerated; a quantification process in which a quantitative value of themenisci is derived on the projection image; and a display controlprocess in which the quantitative value derived by the quantification isdisplayed on the display 6.

By the CPU 11 executing these processes in accordance with the program,the computer functions as an image acquisition unit 21, a plane settingunit 22, a projection unit 23, a quantification unit 24, and a displaycontrol unit 25.

The image acquisition unit 21 acquires the three-dimensional image V0 ofthe knee joint of the test subject from the image storage server 3. Notethat, in a case where the three-dimensional image V0 is already storedin the storage 13, the image acquisition unit 21 may acquire thethree-dimensional image V0 from the storage 13. FIG. 3 is a diagramillustrating the three-dimensional image V0 of the knee joint. Asillustrated in FIG. 3, the three-dimensional image V0 includes a femur30 and a tibia 31. Note that a patella is omitted from FIG. 3 fordescription. There are a cartilage 32 in a portion of the femur 30facing the tibia 31 and a cartilage 33 in a portion of the tibia 31facing the femur 30. In addition, there are menisci 34 between thecartilage 32 and the cartilage 33. In the present embodiment, thethree-dimensional image V0 is an MRI image, and the range of signalvalues (voxel values) in the three-dimensional image V0 differs in eachof a bone region, a cartilage region, a meniscus region, and othermuscle, fat, etc. regions. The image acquisition unit 21 extracts a boneregion and a meniscus region from the three-dimensional image V0 througha threshold process on the signal values. Specifically, a region of arange corresponding to a signal value of a bone is extracted as the boneregion from the three-dimensional image V0. In addition, a region of arange corresponding to a signal value of a meniscus is extracted as themeniscus region from the three-dimensional image V0. Note that the imageacquisition unit 21 may also extract a cartilage region from thethree-dimensional image V0. In this case, the image acquisition unit 21extracts a region of a range corresponding to a signal value of acartilage as the cartilage region from the three-dimensional image V0.The bone region includes the femur 30 and the tibia 31, the cartilageregion includes the cartilages 32 and 33, and the meniscus regionincludes the menisci 34.

The image acquisition unit 21 extracts the bone region, the cartilageregion, and the meniscus region from the three-dimensional image V0 inthe present embodiment, but the present disclosure is not limited tothis. A means that extracts the bone region, the cartilage region, andthe meniscus region from the three-dimensional image V0 may beadditionally provided. Note that in the present embodiment, thecartilage 33 of the tibia 31 is extracted as the cartilage region. Inaddition, extraction of the bone region, the cartilage region, and themeniscus region from the three-dimensional image V0 is not limited tothe threshold process. For example, a determiner that has been subjectedto machine learning by deep learning or the like so as to extract thebone region, the cartilage region, and the meniscus region from thethree-dimensional image V0 may be used.

On the other hand, when the three-dimensional imaging apparatus 2acquires the three-dimensional image V0, the knee joint is imaged in astate where the knee is stretched or slightly bended (10 to 20 degrees).In the three-dimensional image V0, as illustrated in FIG. 3, thedirection in which the femur 30 and the tibia 31 extend from top tobottom is set as z direction, the direction from back to front whenviewing the knee joint from the front is set as y direction, and thedirection from left to right when viewing the knee joint from the frontis set as x direction.

The plane setting unit 22 sets a plane that approximates a joint surfaceof the knee joint as a projection plane for generating a projectionimage, which will be described later, by projecting thethree-dimensional image V0, more specifically, menisci at the knee jointincluded in the three-dimensional image V0. Now, a process performed bythe plane setting unit 22 will be described. First, the plane settingunit 22 sets a minimal three-dimensional region surrounding the tibia 31included in the three-dimensional image V0. FIG. 4 is a diagramillustrating the three-dimensional region. As illustrated in FIG. 4, theplane setting unit 22 sets a minimal three-dimensional region 40surrounding the tibia 31. Note that directions of the sides of thethree-dimensional region 40 are made to match the x direction, the ydirection, and the z direction of the three-dimensional image V0.

Subsequently, the plane setting unit 22 extracts, as a joint surfaceregion, a region excluding a region between condyles from a top surface40A of the three-dimensional region 40. FIG. 5 is a diagram fordescribing extraction of joint surface regions. As illustrated in FIG.5, the plane setting unit 22 sets, as a region 42 between condyles, aregion having a width that corresponds to a predetermined ratio to thelength of a side 41 on the basis of the middle point of the side 41extending in the x direction of the top surface 40A. Note that thepredetermined ratio may be 15 to 25%, preferably 20%, but the presentdisclosure is not limited to this. Then, the plane setting unit 22 sets,as joint surface regions 43 and 44, two regions that are on the left andright of the region 42 between condyles on the top surface 40A. Notethat the joint surface region 43 corresponds to a region on the medialcondyle side in the joint of the tibia 31 and the joint surface region44 corresponds to a region on the lateral condyle side in the joint ofthe tibia 31.

Subsequently, the plane setting unit 22 sets, as the projection plane, aplane that approximates a joint surface included in the joint surfaceregions 43 and 44. Specifically, the plane setting unit 22 derives, asthe projection plane, a plane on which distances from voxelsconstituting the joint surface included in the joint surface regions 43and 44 are minimal by the least squares method. FIG. 6 is a diagram fordescribing deriving of the projection plane. Note that a cross sectionperpendicular to the y axis of the tibia 31 is illustrated in FIG. 6 foreasy description. In addition, only some of the voxels in the jointsurface regions 43 and 44 are represented by the black circles, whilethe distances from the voxels to the plane, that is, a projection plane45, are represented by the arrows. As illustrated in FIG. 6, the planesetting unit 22 derives, as the projection plane 45, a plane on whichthe sum total of the distances from the voxels in the joint surfaceregions 43 and 44 is minimal.

Subsequently, the plane setting unit 22 sets a new projection plane byexcluding a voxel whose distance corresponds to a predetermined ratiofrom the maximum among the distances from the voxels in the jointsurface regions 43 and 44 to the projection plane 45. FIG. 7 is adiagram for describing exclusion of the voxel. Note that FIG. 7illustrates a cross section of the tibia 31 in the zy plane. In thepresent embodiment, the plane setting unit 22 derives the projectionplane 45 on which the distances from the voxels constituting the jointsurface included in the joint surface regions 43 and 44 are minimal.However, on the joint surface, there is a voxel whose distance from theprojection plane 45 is larger than the other voxels, as in a voxel P1illustrated in FIG. 7. If the projection plane is set by using such avoxel, it is not possible to derive the plane that approximates a jointsurface accurately.

Accordingly, the plane setting unit 22 excludes the voxel whose distancecorresponds to the predetermined ratio from the maximum among thedistances from the voxels in the joint surface regions 43 and 44 to theprojection plane 45. Then, by using voxels other than the excludedvoxel, the plane setting unit 22 derives, as a new projection plane 45A,a plane for which the sum total of the distances from the voxels in thejoint surface regions 43 and 44 is minimal. Here, the predeterminedratio may be 5 to 20%, preferably 10%, but the present disclosure is notlimited to this.

In the present embodiment, the plane setting unit 22 repeatedly performsthe above process such that a voxel whose distance corresponds to thepredetermined ratio from the maximum among the distances from the voxelsin the joint surface regions 43 and 44 to the new projection plane 45Ais further excluded and a new projection plane 45B is set, to derive afinal projection plane 46. For example, when the process of setting anew projection plane is repeated twice, in a case where the abovepredetermined ratio is 10%, the ratio of voxels that contribute to thesetting of the projection plane 46 is 81% of all the voxels in the jointsurface regions 43 and 44. Note that the present embodiment assumes thatthe above predetermined ratio is 10% and that the process of setting anew projection plane is repeated twice.

The projection unit 23 generates a projection image by projecting themenisci at the joint in the three-dimensional image V0 in the directionorthogonal to the projection plane 46.

That is, as illustrated in FIG. 8, the projection unit 23 generates aprojection image by projecting the menisci 34 in a direction 47orthogonal to the projection plane 46. Although the tibia 31 is alsoprojected when the projection image is generated in the presentembodiment, only the menisci 34 may be projected to generate theprojection image. Note that “orthogonal” includes, not only a case ofbeing completely orthogonal, but also a case of being orthogonal with anerror of a certain degree, such as about 1 to 2 degrees.

At this time, the projection unit 23 generates the projection image suchthat a line connecting the center of gravity of a joint surface of themedial condyle of the tibia 31 and the center of gravity of a jointsurface of the lateral condyle of the tibia 31 is oriented in apredetermined direction. FIG. 9 is a diagram for describing deriving ofthe centers of gravity. The voxels to be used when deriving theprojection plane 46 is reduced to 81% of all the voxels in the jointsurface regions 43 and 44 by repeating the above-described process ofsetting a new projection plane. In each of the joint surface regions 43and 44, using only the voxels used for the process of setting theprojection plane 46, the projection unit 23 derives the center ofgravity of the joint surface of the medial condyle of the tibia 31 andthe center of gravity of the joint surface of the lateral condyle of thetibia 31. At this time, a center of gravity G1 of the joint surface ofthe medial condyle is derived by using the voxels in the joint surfaceregion 43, while a center of gravity G2 of the joint surface of thelateral condyle is derived by using the voxels in the joint surfaceregion 44. Note that FIG. 9 illustrates regions where the voxels usedfor deriving the center of gravity G1 and the center of gravity G2 arepresent by surrounding the regions by broken lines.

The projection unit 23 generates the projection image by projecting thetibia 31 and the menisci 34 in the direction orthogonal to theprojection plane 46 such that the line connecting the center of gravityG1 and the center of gravity G2 is horizontal in the projection image.FIG. 10 is a diagram illustrating the projection image of the tibia 31and the menisci 34. In a projection image 50 illustrated in FIG. 10, aline 48 connecting center of gravity G1 and the center of gravity G2 ishorizontal. Note that meniscus regions 54A and 54B are hatched in FIG.10. The meniscus region 54A corresponds to the medial meniscus, whereasthe meniscus region 54B corresponds to the lateral meniscus.

Note that an Akagi line may be derived on a joint surface of the tibia31, and the projection image may be generated by projecting the tibia 31and the menisci 34 in the direction orthogonal to the projection plane46 such that the Akagi line is oriented in the perpendicular directionof the projection image. The Akagi line is a line connecting a posteriorcruciate ligament attachment and the medial border of a patellar tendonattachment.

FIG. 11 is a diagram illustrating the projection image generated suchthat the Akagi line is oriented in the perpendicular direction of theprojection image. Note that as illustrated in FIG. 11, in the projectionimage 50, an Akagi line 49 is perpendicular.

Furthermore, the projection unit 23 sets a region of interest in theprojection image 50. In the present embodiment, the projection unit 23sets, as regions of interest, regions corresponding to subchondral boneregions at the joint. FIG. 12 is a diagram for describing setting of theregions of interest. In the projection image 50 illustrated in FIG. 12,for describing setting of the subchondral bone regions, the meniscusregions are omitted, and cartilage regions 51A and 51B are illustrated.In addition, in FIG. 12, each of the cartilage region 51A on the jointsurface of the medial condyle and the cartilage region 51B on the jointsurface of the lateral condyle are hatched.

Here, the subchondral bone regions are regions where the joint of thetibia 31 and the joint of the femur 30 wear each other. The peripheriesof the cartilage regions 51A and 51B in the projection image 50 and thejoint of the femur 30 do not wear each other. Thus, the projection unit23 extracts, as the subchondral bone regions, regions excluding regionsin predetermined ranges from the edges of the cartilage regions 51A and51B in the projection image 50, and sets the extracted subchondral boneregions as a region of interest 52A and a region of interest 52B. On theother hand, in the tibia 31, an outline that defines a region in which acartilage is supposed to be present in the joint is included in thejoint surface as a protruding portion. Thus, the projection unit 23 mayset the region of interest 52A and the region of interest 52B byregarding, as the cartilage regions 51A and 51B, the regions surroundedby the protruding portions on the joint surface. In addition, theprojection unit 23 may include a determiner that has been subjected tomachine learning by deep learning or the like so as to extract thesubchondral bone regions from the projection image 50, and thesubchondral bone regions may be extracted by using the determiner.

The quantification unit 24 derives quantitative values of the meniscusregions 54A and 54B in the projection image 50. A quantitative value isderived for each of the medial meniscus and the lateral meniscus in thepresent embodiment, but the present disclosure is not limited to this.The quantitative value may be derived by combining the medial meniscusand the lateral meniscus together.

FIG. 13 is a diagram for describing deriving of the quantitative values.First, the quantification unit 24 derives the areas of the region ofinterest 52A and the region of interest 52B and the areas of themeniscus regions 54A and 54B. The quantification unit 24 further derivesthe areas of meniscus regions 54A and 54B within the region of interest52A and the region of interest 52B. The areas of meniscus regions 54Aand 54B within the region of interest 52A and the region of interest 52Bare the areas of in-region-of-interest meniscus regions 55A and 55Bhatched in FIG. 13. Here, the area of each pixel is known in theprojection image 50. Thus, the quantification unit 24 counts the numberof pixels in the region of interest 52A and the region of interest 52Band the meniscus regions 54A and 54B, and multiplies the counted numberof pixels by the area per pixel, thereby deriving areas S1A and S1B ofthe region of interest 52A and the region of interest 52B and areas S2Aand S2B of the meniscus regions 54A and 54B. The quantification unit 24further derives areas S3A and S3B of the meniscus regions 54A and 54Bwithin the region of interest 52A and the region of interest 52B, thatis, the in-region-of-interest meniscus regions 55A and 55B. Note thateach of the areas S3A and S3B of the in-region-of-interest meniscusregions 55A and 55B is one of the quantitative values. Also, each of theareas S2A and S2B of the meniscus regions 54A and 54B is one of thequantitative values.

In the present embodiment, the quantification unit 24 derives, as one ofthe quantitative values, each of coverages of the meniscus regions 54Aand 54B in the region of interest 52A and the region of interest 52B.The coverage of the meniscus region 54A in the medial region of interest52A is derived as S3A/S1A, which is the area S3A of thein-region-of-interest meniscus region 55A to the area S1A of the regionof interest 52A. The coverage of the meniscus region 54B in the lateralregion of interest 52B is derived as S3B/S1B, which is the area S3B ofthe in-region-of-interest meniscus region 55B to the area S1B of theregion of interest 52B.

FIG. 14 is a diagram illustrating a display screen of the quantitativevalues of the menisci. As illustrated in FIG. 14, on a display screen 60of the quantitative values of the menisci, the projection image 50, acoverage 61 of the medial meniscus, and a coverage 62 of the lateralmeniscus are displayed. The display screen 60 enables an operator tounderstand, in addition to the state of the menisci 34, that thecoverage 61 of the medial meniscus is 0.548 and the coverage 62 of thelateral meniscus is 0.479.

Note that over-time observation may be performed for the same testsubject by comparing a plurality of three-dimensional images whoseimaging timings are different. In such a case, in a case where aprojection image is generated from a first three-dimensional image V1whose imaging timing is in the past, a projection image is preferablygenerated from a second three-dimensional image V2 whose imaging timingis new for over-time observation. Note that the first three-dimensionalimage V1 corresponds to another three-dimensional image according to thepresent disclosure. In this case, although the menisci 34 may wear orbecome deformed over time, the shape of the joint does not becomedeformed. Thus, upon generating the projection image from the firstthree-dimensional image V1, information representing the projectionplane 46 is preferably stored in the image storage server 3, and theprojection image from the second three-dimensional image V2 ispreferably generated by acquiring the information of the projectionplane stored for the same test subject and using the acquiredinformation of the projection plane.

This can reduce the calculation amount when the projection image fromthe second three-dimensional image V2 is generated. In addition, in thiscase, in a case where the region of interest 52A and the region ofinterest 52B are set in the projection image from the firstthree-dimensional image V1, the same regions of interest as those in theprojection image from the first three-dimensional image V1 arepreferably set in the projection image from the second three-dimensionalimage V2. Thus, the quantitative values of the menisci 34 in the firstthree-dimensional image V1 and the quantitative values of the menisci 34in the second three-dimensional image V2 can be temporally compared witheach other with ease.

FIG. 15 is a diagram illustrating a display screen for temporalcomparison of the quantitative values of the menisci. As illustrated inFIG. 15, on a display screen 65 for temporal comparison, a previousprojection image 66 acquired during a previous examination and a currentprojection image 67 acquired during a current examination are displayed.Below the previous projection image 66, an imaging date of thethree-dimensional image and coverages (a coverage of the medial meniscusand a coverage of the lateral meniscus) 68 of the meniscus regions inthe regions of interest acquired from the previous projection image 66are displayed. Below the current projection image 67, an imaging date ofthe three-dimensional image and coverages (a coverage of the medialmeniscus and a coverage of the lateral meniscus) 69 of the meniscusregions in the regions of interest acquired from the current projectionimage 67 are displayed. The display screen 65 enables an operator tounderstand over-time changes in the menisci 34. In FIG. 15, the imagingdate is displayed, however, an imaging time may also be displayed inaddition to the imaging date.

Note that the region of interest 52A and the region of interest 52B arenot limited to the subchondral bone regions. As illustrated in FIG. 16,the cartilage regions may also be set as a region of interest 56A and aregion of interest 56B. In this case, the projection image 50 includingthe meniscus region too is as illustrated in FIG. 17. Note that part ofthe cartilage is lost on the region of interest 56A side. In a casewhere coverages of the meniscus regions in the regions of interest areto be derived as the quantitative values in this case, it is assumedthat the area of the region of interest 56A is S4A, the area of theregion of interest 56B is S4B, the areas of the meniscus regions 54A and54B within the region of interest 56A and the region of interest 56B areS5A and SSB. Thus, the coverage of the meniscus region 54A in the medialregion of interest 56A is derived as S5A/S4A. The coverage of themeniscus region 54B in the lateral region of interest 56B is derived asS5B/S4B.

In addition, although the projection plane is set on the basis of thetibia 31, the projection plane may alternatively be set on the basis ofthe femur 30. In this case, for example, the projection plane may be setsuch that the direction in which the central axis of the femur extendsis the projection direction.

The quantification unit 24 may also derive the volumes of the menisci 34as the quantitative values. In this case, the quantification unit 24 mayderive the volumes of the menisci 34 by counting the number of pixels ofthe meniscus regions 54A and 54B in the three-dimensional image andmultiplying the number of pixels by the volume per pixel.

The quantification unit 24 may also derive the thicknesses of themenisci 34 as the quantitative values. FIG. 18 is a diagram fordescribing deriving of the thickness of a meniscus. As illustrated inFIG. 18, the quantification unit 24 sets each of reference points O1 andO2 in the projection image 50. The reference point O1 is a point atwhich the average distance therefrom to the nearest edge of the meniscusregion 54A is minimal, but the present disclosure is not limited tothis. The reference point O2 is a point at which the average distancetherefrom to the nearest edge of the meniscus region 54B is minimal, butthe present disclosure is not limited to this. Then, from the referencepoint O1 as the center, the quantification unit 24 sets a plurality ofreference lines L1-1, L1-2 . . . L1-n radially at equiangular intervalsas for the medial meniscus region 54A. Then, by using thethree-dimensional image V0, the quantification unit 24 derives theaverage thickness of a meniscus 34 on each reference line. Note that theplurality of reference lines may be set at, for example, 10-degreeintervals, but the present disclosure is not limited to this.

FIG. 19 is a cross sectional view of the meniscus for describingderiving of the average thickness of the meniscus. Note that FIG. 19illustrates a cross section of the meniscus 34 on a certain referenceline. As illustrated in FIG. 19, the meniscus 34 has a wedge shape incross section. Thus, as illustrated by the arrows in FIG. 19, thequantification unit 24 derives the thicknesses of the meniscus 34 at aplurality of positions on the reference line, and derives the average ofthe plurality of thicknesses as the thickness of the meniscus 34 on thereference line. Furthermore, the quantification unit 24 derives theaverage of the thicknesses of the meniscus 34 on the plurality ofreference lines as the thickness of the meniscus 34. Note that only thethickness of the meniscus 34 in the medial meniscus region 54A isderived in FIG. 18 and FIG. 19, but the meniscus 34 in the lateralmeniscus region 54B may be derived in the same manner as that for themeniscus 34 in the medial meniscus region 54A.

Note that the thickness of the meniscus 34 on each reference line is oneof the quantitative values. Each of the plurality of thicknesses of themeniscus 34 derived on each reference line is also one of thequantitative values. A representative value such as the average, median,minimum, or maximum of the thicknesses of the meniscus 34 related to allthe reference lines is also one of the quantitative values.

The quantification unit 24 may also divide the meniscus regions 54A and54B into a plurality of regions and may derive a quantitative value ineach of the divided regions. FIG. 20 is a diagram for describingdividing of the meniscus regions. As illustrated in FIG. 20, thequantification unit 24 sets the reference points O1 and O2, which aresubstantially the same as those in FIG. 18, in the projection image 50.Then, the quantification unit 24 sets a reference line L31 extending inthe X direction with the reference point O1 as the origin, and furthersets, with the reference point O1 as the origin, reference lines L32 andL33 at which the angles with the reference line L31 are 120 degrees and240 degrees, respectively, counterclockwise. In addition, thequantification unit 24 sets a reference line L41 extending in the Xdirection with the reference point O2 as the origin, and further sets,with the reference point O2 as the origin, reference lines L42 and L43at which the angles with the reference line L41 are 120 degrees and 240degrees, respectively, clockwise. Then, the quantification unit 24divides the meniscus region 54A into a front section 54A-F defined bythe reference lines L31 and L32, a middle section 54A-M defined by thereference lines L32 and L33, and a back section 54A-B defined by thereference lines L33 and L31. In addition, the quantification unit 24divides the meniscus region 54B into a front section 54B-F defined bythe reference lines L41 and L42, a middle section 54B-M defined by thereference lines L42 and L43, and a back section 54B-B defined by thereference lines L43 and L41.

Then, as for the meniscus region 54A, the quantification unit 24 derivesthe quantitative value for each of the front section 54A-F, the middlesection 54A-M, and the back section 54A-B. In addition, as for themeniscus region 54B, the quantification unit 24 derives the quantitativevalue for each of the front section 54B-F, the middle section 54B-M, andthe back section 54B-B. Note that the area, volume, and thickness in themeniscus regions 54A and 54B can be used as the quantitative value ofeach divided region. In addition, the coverage in the meniscus regions54A and 54B within the regions of interest may also be derived as thequantitative value of each divided region.

On the other hand, the menisci 34 may move out of the region of interestdue to a load or impact. The term “out of the region of interest” meansa more medial area for the medial region of interest 52A, while the term“out of the region of interest” means a more lateral area for thelateral region of interest 52B. Thus, in the present embodiment, thequantification unit 24 may derive quantitative values of the menisci 34that are out of the regions of interest. FIG. 21 is a diagram fordescribing deriving of the quantitative values of the menisci 34 thatare out of the regions of interest. As illustrated in FIG. 21, thequantification unit 24 sets the reference points O1 and O2, which aresubstantially the same as those in FIG. 18, in the projection image 50.Then, as for the medial meniscus region 54A, the quantification unit 24determines an out-of-region-of-interest meniscus region 57A that sticksout to the medial side of the knee from the region of interest 52A onthe basis of a line 59A perpendicular to the reference point O1 in theprojection image 50. In addition, as for the lateral meniscus region54B, the quantification unit 24 determines an out-of-region-of-interestmeniscus region 57B that sticks out to the lateral side of the knee fromthe region of interest 52B on the basis of a line 59B perpendicular tothe reference point O2 in the projection image 50. Theout-of-region-of-interest meniscus regions 57A and 57B are hatched inFIG. 21. Then, the quantification unit 24 derives the quantitativevalues of the menisci 34 in the out-of-region-of-interest meniscusregions 57A and 57B. As the quantitative values, the areas, the volumes,and the thicknesses of the out-of-region-of-interest meniscus regions57A and 57B can be used. The thicknesses may be representative values ofthe thicknesses of the menisci 34 in the out-of-region-of-interestmeniscus regions 57A and 57B or may be the thicknesses of the menisci 34at pixel positions of the out-of-region-of-interest meniscus regions 57Aand 57B on the projection image 50.

If the thicknesses of the menisci 34 at pixel positions of theprojection image 50 are derived as the quantitative values, thequantification unit 24 may derive a thickness map of the thicknesses ofthe menisci 34 as the quantitative values. FIG. 22 is a diagramillustrating a thickness map of the menisci. As illustrated in FIG. 22,in a thickness map 70, a distribution of the thicknesses of the meniscusregions 54A and 54B in the projection image is illustrated by 6-levelcolors. In the thickness map 70, the darker the color is, the thickerthe meniscus regions 54A and 54B are. Note that the color differencesare represented by hatching differences in FIG. 22. In addition, thethickness map 70 includes a reference 71 indicating a relationshipbetween the color and the thickness. By referring to the reference 71,it is possible to visually recognize the distribution of the thicknessesof the meniscus regions 54A and 54B in the thickness map 70 with ease.

Note that the derived quantitative values are transmitted to the imagestorage server 3 and stored therein in association with thethree-dimensional image V0 together with information such as thepatient's name, an imaging date, the positions of the region of interest52A and the region of interest 52B, and the projection image 50. Inaddition to the imaging date, an imaging time may also be stored on theimage storage server 3.

The display control unit 25 causes the display 6 to display theprojection image 50 together with the quantitative values.

Next, a process performed in the present embodiment will be described.FIG. 23 is a flowchart illustrating the process performed in the presentembodiment. First, the image acquisition unit 21 acquires thethree-dimensional image V0 (step ST1) and extracts a bone region and ameniscus region from the three-dimensional image V0 (step ST2). Notethat, as described above, the image acquisition unit 21 may also extracta cartilage region. Subsequently, the plane setting unit 22 sets a planethat approximates a joint surface of a joint of the tibia 31 as theprojection plane 46 for generating a projection image by projecting thethree-dimensional image V0 (step ST3).

Subsequently, the projection unit 23 generates the projection image 50by projecting a meniscus 34 at the joint in the three-dimensional imageV0 in the direction orthogonal to the projection plane 46 (step ST4).Then, the quantification unit 24 derives a quantitative value of themeniscus 34 in the projection image 50 (step ST5). Furthermore, thedisplay control unit 25 causes the display 6 to display the projectionimage 50 and the quantitative value (step ST6), and the process ends.

In the above manner, in the present embodiment, the plane thatapproximates the joint surface of the joint is set as the projectionplane 46 for generating the projection image by projecting thethree-dimensional image V0. This can prevent a lost portion of themeniscus from being projected as if the meniscus is present, inparticular, as for a joint surface of a comparatively flat joint, suchas the tibia 31. Thus, according to the present embodiment, theprojection plane for generating the projection image 50 of the meniscuscan be set appropriately from the three-dimensional image V0 includingthe joint.

Note that the projection unit 23 generates the two-dimensionalprojection image 50 in the above embodiment, but the projection unit 23may generate a three-dimensional projection image. FIG. 24 is a diagramillustrating a three-dimensional projection image. As illustrated inFIG. 24, a projection image 80 includes, as in the above embodiment,meniscus regions 81A and 81B of the tibia 31 and cross-sectional views82A and 82B illustrating the thicknesses of the meniscus regions 81A and81B. Note that the positions of cross sections in the cross-sectionalviews 82A and 82B may be changeable. For example, the positions of crosssections may be changeable radially from the centers that are thereference points O1 and O2 in FIG. 18. Also for the three-dimensionalprojection image 80 generated in this manner, the quantitative valuescan be derived and displayed in the above-described manner.

In addition, in the above embodiment, when generating projection imagesfrom three-dimensional images V0 of a plurality of different testsubjects, the same projection plane may be set. In this case, theprojection plane for further generating projection images is preferablythe same in the three-dimensional images V0 of the plurality ofdifferent test subjects. Thus, states of the menisci of the testsubjects can be compared with each other with ease.

Furthermore, in the above embodiment, the areas, volumes, thicknesses,and representative values of the thicknesses of the menisci 34, thecoverages of the in-region-of-interest meniscus regions 55A and 55B, andthe areas, volumes, thicknesses, or the like of theout-of-region-of-interest meniscus regions 57A and 57B are derived asthe quantitative values. However, any one or any combination of thesequantitative values may also be derived.

In addition, the projection plane is set by using both the joint surfaceregions 43 and 44 on the joint surface of the medial condyle and thejoint surface of the lateral condyle in the above embodiment, but thepresent disclosure is not limited to this. The projection plane may beset by using only the joint surface region 43 on the joint surface ofthe medial condyle. In this case, by using the set projection plane, theprojection image of the joint surface region 43 on the joint surface ofthe medial condyle may be generated. On the other hand, the projectionplane may be set by using only the joint surface region 44 on the jointsurface of the lateral condyle. In this case, by using the setprojection plane, the projection image of the joint surface region 44 onthe joint surface of the lateral condyle may be generated.

Furthermore, the process of setting a new projection plane by excludinga voxel is performed repeatedly in the above embodiment, but the presentdisclosure is not limited to this. On the initially set projection plane45, voxels whose distance corresponds to the predetermined ratio fromthe maximum among distances from the voxels in the joint surface regions43 and 44 to the projection plane 45 may be excluded to set a finalprojection plane. In this case, the predetermined ratio may be the sameas, or larger than, that in a case where the process of setting a newprojection plane is performed a plurality of times by excluding a voxel.For example, the predetermined ratio may be 20%.

In addition, in the above embodiment, for example, as a hardwareconfiguration of a processing unit that executes various processes, suchas the image acquisition unit 21, the plane setting unit 22, theprojection unit 23, the quantification unit 24, and the display controlunit 25, various processors below can be used. The various processorsinclude, in addition to a CPU, which is a general-purpose processor thatfunctions as various processing units by executing software (programs)as described above, a programmable logic device (PLD), which is aprocessor in which the circuit configuration is changeable aftermanufacture, such as an FPGA (Field Programmable Gate Array), adedicated electric circuit, which is a processor having a circuitconfiguration that is specially designed to execute specific processing,such as an ASIC (Application Specific Integrated Circuit), and the like.

One processing unit may be constituted by one of these variousprocessors or may be constituted by two or more processors of the sametype or different types in combination (e.g., a combination of aplurality of FPGAs or a combination of a CPU and an FPGA). In addition,a plurality of processing units may be constituted by one processor.

As a first example for constituting a plurality of processing units byone processor, one processor may be constituted by a combination of oneor more CPUs and software, and this processor may function as aplurality of processing units, as typified by a computer such as aclient or a server. As a second example, a processor may be used thatimplements the functions of the entire system including a plurality ofprocessing units with one IC (Integrated Circuit) chip, as typified by asystem on chip (SoC) or the like. In this manner, various processingunits are constituted by one or more of the above various processors interms of hardware configuration.

More specifically, the hardware configuration of these variousprocessors may be electric circuitry constituted by combining circuitelements such as semiconductor elements.

REFERENCE SIGNS LIST

1 meniscus projection plane setting apparatus

2 three-dimensional imaging apparatus

3 image storage server

4 network

6 display

7 input device

11 CPU

12 memory

13 storage

14 communication I/F

21 image acquisition unit

22 plane setting unit

23 projection unit

24 quantification unit

25 display control unit

30 femur

31 tibia

32, 33 cartilage

34 meniscus

40 three-dimensional region

40A top surface

41 side

42 region between condyles

43, 44 joint surface region

45, 45A, 45B, 46 projection plane

47 direction orthogonal to projection plane

48 line connecting centers of gravity

49 Akagi line

50 projection image

51A, 51B, 56A, 56B cartilage region

52A, 52B, 56A, 56B region of interest

54A, 54B, 81A, 81B meniscus region

54A-F, 54B-F front section

54A-M, 54B-M middle section

54A-B, 54B-B back section

55A, 55B in-region-of-interest meniscus region

57A, 57B out-of-region-of-interest meniscus region

59A, 59B line

60, 65 display screen

61, 62, 68, 69 coverage

66 previous projection image

67 current projection image

70 thickness map

71 reference

80 projection image

82A, 82B cross-sectional view

90 tibia

91A, 91B meniscus

92 lost portion

93 projection image

C0 central axis

G1, G2 center of gravity

L1-1, L1-2 L1-n, L31, L32, L33, L41, L42, L43 reference line

P1 voxel

O1, O2 reference point

V0, V1, V2 three-dimensional image

What is claimed is:
 1. A meniscus projection plane setting apparatuscomprising at least one processor configured to: acquire athree-dimensional image of a knee joint; and set a plane thatapproximates a joint surface of the knee joint as a projection plane forgenerating a projection image by projecting a meniscus at the knee jointincluded in the three-dimensional image.
 2. The meniscus projectionplane setting apparatus according to claim 1, wherein the processor isconfigured to exclude, from among pixel positions on the joint surface,a pixel position whose distance from the projection plane is greaterthan or equal to a predetermined threshold and set a new projectionplane.
 3. The meniscus projection plane setting apparatus according toclaim 2, wherein the processor is configured to set the projection planeby repeating excluding of the pixel position and setting of the newprojection plane a plurality of times.
 4. The meniscus projection planesetting apparatus according to claim 1, wherein the processor isconfigured to set the plane that approximates the joint surface by aleast squares method.
 5. The meniscus projection plane setting apparatusaccording to claim 1, wherein the knee joint is a tibial joint.
 6. Themeniscus projection plane setting apparatus according to claim 5,wherein the processor is configured to extract, as a joint surfaceregion, a region excluding a region between condyles from an image inwhich a joint surface of the tibia is viewed in a predetermineddirection and set, as the projection plane, a plane that approximatesthe joint surface included in the joint surface region.
 7. The meniscusprojection plane setting apparatus according to claim 1, wherein theprocessor is configured to generate the projection image by furtherprojecting the meniscus at the knee joint in the three-dimensional imagein a direction orthogonal to the projection plane.
 8. The meniscusprojection plane setting apparatus according to claim 5, wherein theprocessor is configured to generate the projection image by furtherprojecting the meniscus at the knee joint in the three-dimensional imagein a direction orthogonal to the projection plane such that a lineconnecting a center of gravity of a joint surface of a medial condyle ofthe tibia and a center of gravity of a joint surface of a lateralcondyle of the tibia is oriented in a predetermined direction.
 9. Themeniscus projection plane setting apparatus according to claim 5,wherein the processor is configured to generate the projection image byfurther projecting the meniscus at the knee joint in thethree-dimensional image in a direction orthogonal to the projectionplane such that an Akagi line at the tibia when viewed in a directionperpendicular to the projection plane is oriented in a predetermineddirection.
 10. The meniscus projection plane setting apparatus accordingto claim 7, wherein the processor is configured to further cause adisplay to display the projection image.
 11. The meniscus projectionplane setting apparatus according to claim 7, wherein the processor isconfigured to further derive a quantitative value of the meniscus in theprojection image.
 12. The meniscus projection plane setting apparatusaccording to claim 11, wherein the processor is configured to derive, asthe quantitative value, at least one of an area of the meniscus, avolume of the meniscus, a lost area of the meniscus, a representativevalue of a thickness of the meniscus, or thicknesses of the meniscus atpositions of the projection image.
 13. The meniscus projection planesetting apparatus according to claim 12, wherein the processor isconfigured to generate a thickness map of the meniscus in a case ofderiving the thicknesses of the meniscus at the positions of theprojection image.
 14. The meniscus projection plane setting apparatusaccording to claim 13, wherein the processor is configured to furthercause a display to display the thickness map.
 15. The meniscusprojection plane setting apparatus according to claim 11, wherein theprocessor is configured to derive the quantitative value of the meniscuswithin or out of a region of interest at the knee joint in thethree-dimensional image in the projection image.
 16. The meniscusprojection plane setting apparatus according to claim 15, wherein theregion of interest is a region where a subchondral bone region or acartilage is supposed to be present at the knee joint.
 17. The meniscusprojection plane setting apparatus according to claim 15, wherein theprocessor is configured to set, in a case where there is a derivedresult of another quantitative value derived from anotherthree-dimensional image whose imaging timing is different for a sametest subject as a test subject for which the three-dimensional image isacquired, the region of interest at a same position as a position of theregion of interest in deriving the other quantitative value.
 18. Themeniscus projection plane setting apparatus according to claim 15,wherein the processor is configured to derive, as the quantitativevalue, a coverage of the meniscus in the region of interest.
 19. Themeniscus projection plane setting apparatus according to claim 15,wherein the processor is configured to derive, as the quantitativevalue, at least one of a volume of the meniscus out of the region ofinterest, an area of the meniscus out of the region of interest, or athickness of the meniscus out of the region of interest.
 20. Themeniscus projection plane setting apparatus according to claim 11,wherein the processor is configured to divide the meniscus in theprojection image into a plurality of regions and derive the quantitativevalue in each of the regions obtained by dividing.
 21. The meniscusprojection plane setting apparatus according to claim 7, wherein theprocessor is configured to set, in a case where there is anotherprojection image generated from another three-dimensional image whoseimaging timing is different for a same test subject as a test subjectfor which the three-dimensional image is acquired, a projection plane,by which the other projection image is generated, as the projectionplane for generating the projection image by projecting thethree-dimensional image.
 22. A meniscus projection plane setting methodcomprising: acquiring a three-dimensional image of a knee joint; andsetting a plane that approximates a joint surface of the knee joint as aprojection plane for generating a projection image by projecting ameniscus at the knee joint included in the three-dimensional image. 23.A non-transitory computer readable recording medium storing a meniscusprojection plane setting program for causing a computer to execute:acquiring a three-dimensional image of a knee joint; and setting a planethat approximates a joint surface of the knee joint as a projectionplane for generating a projection image by projecting a meniscus at theknee joint included in the three-dimensional image.